The first and third substrates may be produced using microelectronic techniques, with operations involving diffusion, deposition of metal layers, and etching of these layers. The intermediate substrate is also produced with these type of operations, but in addition, given that its main function is mechanical, it is produced by deep etching operations for the purpose of cutting out a micromechanical structure with very fine features and very small thicknesses.
Typically, the machining of the moving structure may be carried out using, as intermediate substrate 2, a silicon-on-insulator (SOI) substrate, but other methods are also possible. A silicon-on-insulator substrate consists of a fixed monolithic silicon substrate 5 a few hundred microns in thickness, which carries on its front face a thin silicon oxide layer 6 covered with a single-crystal silicon layer 7 a few tens of microns in thickness. The machining consists in etching the single-crystal silicon 7 via its front face until the oxide layer is reached, using a selective etchant that etches the silicon without significantly etching the oxide. The etching is stopped when the oxide layer 6 is bared. This oxide layer 6 may itself be removed by selective etching with another etchant so as to preserve only the surface silicon layer 7. This may be then be etched into the desired surface patterns by means of photoetching techniques commonly used in microelectronics, in order thus to obtain the desired moving planar structure.
Given that the moving planar structure does not “float” relative to the body of the gyrometer, that is to say relative to the fixed monolithic silicon substrate 5, it is necessary for it to be anchored to the monolithic substrate at anchoring points that do not disturb the mobility of the moving planar structure.
In the rest of the description, the term “anchoring feet” will denote the points for anchoring the elements set in movement by the forces that depend on the angular velocity to be measured, the term “anchoring point” corresponding to the excitation elements. When, as will be seen later, the excitation elements for exciting the structure are also set in movement by the forces that depend on the angular velocity to be measured the term “anchoring foot” will apply instead.
The intermediate substrate 2 is fastened to the third substrate 3, for example by means of a polymeric adhesive 4 or by a fusion-bonded joint uniformly distributed between the fixed monolithic substrate 5 and the third substrate 3. This third substrate 3 will also be called the “support substrate”.
The gyrometer 100 is intended to be placed on board an aircraft, a moving vehicle or, more generally, on board a moving system.
It will firstly be recalled that the structure of a high-performance gyrometer produced using these techniques generally comprises two moving masses that are excited in vibration and connected as a tuning fork, that is to say the two masses are connected to a central coupling structure that transfers the vibration energy from the first mass to the second mass, and vice versa.
The masses are excited in vibration in the plane of the structure by electrostatic forces applied via interdigitated electrode combs. This vibration in the plane of the structure is exerted perpendicular to an axis called the “sensitive axis” of the gyrometer, perpendicular to the direction of this vibration. When the gyrometer rotates at a certain angular velocity about its sensitive axis, the composition of the forced vibration with the angular rotation vector generates, by the Coriolis effect, forces that set the moving masses into natural vibration perpendicular to the excitation vibration and to the axis of rotation; the amplitude of this natural vibration is proportional to the speed of rotation. The two masses vibrate in phase opposition so as to obtain a tuning-fork effect.
The natural vibration is detected by an electrical detection structure. The electrical signals that result therefrom are used to deduce from them a value of the angular velocity about the sensitive axis.
In certain cases the sensitive axis lies in the plane of the wafer and the detection structure detects a movement perpendicular to the plane of the moving masses. In other cases, the sensitive axis of the gyrometer is the axis Oz perpendicular to the plane of the wafer. The excitation movement of the moving masses is generated in a direction Ox of the plane, while a movement resulting from the Coriolis force is detected in a direction Oy, perpendicular to Ox, in the same plane.
On a true tuning fork, the vibrating structure is fastened to the external environment via a single point, which is a vibration node of the vibrating structure. This allows the vibration energy to be confined within the vibrating structure and thus makes it possible to achieve high quality factors.
On a planar substrate, the vibrating structure is in general fastened by several distant anchoring feet. However, these anchoring feet are not always vibration nodes of the vibrating structure, so that some of the vibratory energy is transmitted to the external environment via the substrate.
One important objective of the invention is therefore to prevent this loss of energy so as to improve the tuning-fork effect.